In the 1980s, C. W. Tang disclosed a double-layer OLED (Organic Light-Emitting Device) (U.S. Pat. No. 4,356,429; Appl. Phys. Lett. 1987, 51, 12, 913). This finding is based on a multilayer structure comprising an emissive electron-transporting layer and a hole transport layer of appropriate organic material. Alq3 (q: deprotonated 8-hydroxythionyl)) was used as emissive electron-transporting material. Since then, the material OLED used in continuous studies of OLED has the following advantages: (1) low operating voltage, (2) thin overall structure, (3) emitted light rather than modulated light, (4) good light-emitting efficiency, (5) panchromatic potential, and high contrast and resolution. These advantages suggest that OLEDs may be used in flat panel displays.
Organic small molecules are studied to improve the performance of OLEDs. Typically, phosphorescent materials are used as light emitters in the light-emitting layer of the OLED, but in different phosphorescent materials, complexes with iridium and platinum are still the dominant materials. Because the iridium-based material has octahedral geometry configuration, the OLED made by iridium-based material has high performance, in addition, it has no great efficiency attenuation. In contrast, platinum-based materials have a planar geometry, so OLEDs made from platinum-based materials have great efficiency attenuation although they have high performance; in other words, these devices can achieve high performance only under very low brightness, and at normal operating brightness levels, for example, below 1000 cd m−2, the performance of devices will be lowered to a very low level. For example, we developed a class of platinum-containing phosphorescent materials in 2007 and devices made by such materials can achieve performance as high as 51.8 cd A−1, but their performance drastically dropped to 50% of the highest performance (Appl. Phys. Lett. 91, 2007, 063508); therefore, only the iridium-based materials can be used in OLED products now.
In general, materials with high quenching constant (above 108 dm3 mol−1 s−1) may cause severe triplet and triplet annihilation cause rapid efficiency attenuation. In addition to efficiency attenuation, devices made by platinum-based materials can be operated in a very narrow doping window due to high quenching constant. In other words, devices with high efficiency and high color purity can only be achieved in a very small doping range (e.g. 1% to 2%), thus, platinum-based materials are not accepted in the industries.
In order to solve this problem, different researchers propose different schemes and prepare different materials. In 2010, we added a large group, Huo added a nonplanar base, and in 2012, Xie added a nonplanar base to a platinum-based material (Chem. Eur. J. 2010, 16, 233-247; Inorg. Chem. 2010, 49, 5107-5119; Chem. Commun. 2012, 48, 3854-3856). However, this problem has not been significantly improved, and the devices prepared still have over 50% efficiency attenuation, which demonstrated that the addition of large groups and non-planar groups are not usually a feasible solution to this problem.